The study is intended to isolate the water permeable membrane units that make the urinary plasma membrane permeable to water in response to vasopressin. Because it is well characterized and easy to work with, the toad urinary bladder is used as the source of the channels. Membrane proteins are labeled covalently with radioactive iodine to identify proteins in the plasma membrane in the presence but not in the absence of stimulation by vasopressin. Following withdrawal of vasopressin, endocytosed membrane is labeled using an intracellular labeling technique. Finally, a cell fraction enriched in endocytosed membrane is isolated and used to prepare monoclonal antibodies.